This invention relates to central station alarm systems, and more particularly to central station alarm systems in which alarm signals are transmitted from one or more remote stations to a central station by radio signals.
A typical central station alarm system includes a central station and a plurality of remote stations for transmitting alarm signals to the central station. Each remote station monitors one or more possible alarm conditions (e.g., smoke, fire, unauthorized entry, refrigeration loss, etc.) at the location of the remote station and produces an output signal identifying the remote station and type of alarm condition when an alarm condition occurs. The central station receives and decodes the signals transmitted by the remote stations and initiates action appropriate to the alarm conditions which are occurring. For example, the central station may alert the operator of the system to the occurrence of an alarm condition by displaying information which identifies the remote station signaling the alarm condition and the type of alarm condition detected by that remote station.
Although referred to for convenience herein as central station alarm systems, systems of this same type may be used for monitoring conditions of any kind at remote locations. For example, pressures at a number of points throughout a pipeline system can be monitored using this type of system. The proper functioning of remotely located automatic oil or other wells can be similarly monitored with this type of system. Many other similar applications will occur to those familiar with such systems.
The remote stations may be connected to the central station via leased (e.g., telephone) or private wire connections. In many areas, however, wire connections are unavailable, unreliable, or uneconomical. In remote or relatively undeveloped areas, telephone wires may not be available and it may be uneconomical to run wires just for an alarm system. Similarly, in highly congested urban areas, spare telephone wires may not be available and the cost of running additional wires may be prohibitive. There are also many areas where telephone or other existing wire communications networks are not sufficiently reliable for use in transmitting alarm signal information, and again the cost of installing private wire connections may be prohibitive. There has therefore been increasing interest in central station alarm systems in which the alarm signals are transmitted to the central station by radio communication.
In a typical radio central station alarm system, a remote station which detects an alarm condition broadcasts a radio frequency signal identifying the remote station and the type of alarm condition occurring at that remote station. A central station located within range of the remote station receives the radio signal and determines the identity of the remote station and type of alarm condition causing that signal to be transmitted. All of the remote stations in a particular area usually operate on the same frequencies. This is done to conserve band space and may in addition be required by official regulations. This also makes it possible to construct central station receivers which are responsive to a relatively narrow band of radio frequencies, as is usually desirable for economic reasons and to minimize noise and other interference. However, this also means that the central station may receive signals from two or more remote stations simultaneously. It is therefore necessary to provide a system in which the probability that signals transmitted by one remote station will be completely obscured by signals transmitted by other remote stations is acceptably low. It is also necessary to provide a system which does not respond to mixed signals from two or more remote stations to produce erroneous alarm indications. It is also desirable to insure that no remote station can transmit continuously for extended periods of time and thereby obscure signals transmitted by other remote stations.
To minimize the possibility of false or erroneous alarm indications, it is desirable to use a simple format for the alarm signals, i.e., a format in which information can be accurately and reliably encoded and decoded. It is also desirable for each remote station to transmit each alarm signal a number of times to insure that the signal is correctly received. These signals are preferably transmitted during several spaced time intervals to reduce the probability that they will be completely obscured by other alarm signals. Since noise may be present on the frequencies used by the system, it is desirable that the system have a high immunity to noise.
In view of the foregoing, it is an object of this invention to improve and simplify radio central station alarm systems.
It is a more particular object to provide improved remote station transmitter apparatus for use in radio central station alarm systems.
It is another more particular object of this invention to provide improved central station receiver apparatus for use in radio central station alarm systems.